Inexpensive variable rep-rate source for high-energy, ultrafast lasers

ABSTRACT

System for converting relatively long pulses from rep-rate variable ultrafast optical sources to shorter, high-energy pulses suitable for sources in high-energy ultrafast lasers. Fibers with positive group velocity dispersion (GVD) and self phase modulation are advantageously employed with the optical sources. These systems take advantage of the need for higher pulse energies at lower repetition rates so that such sources can be cost effective.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to rep-rate variable, ultrafast opticalsources that have energies suitable for applications in high-energy,ultrafast laser systems. These sources may replace mode-locked lasersfor generating short pulses for higher energy applications. The sourcesare based on inexpensive longer pulse sources and utilize pulsecompression in optical fibers to obtain shorter pulses at high pulseenergies.

2. Description of the Related Art

The present invention relates to a source for high-energy, ultrafastlasers. Most of the related art is focused on sources fortelecommunications. Presently, there is a push to develop technologythat allows an increase in the data to be transmitted by a fiber fortelecommunications. There are two means for increasing the data rate.One is by increasing the number of channels, where the channels are atdifferent wavelengths (WDM). The other is by increasing the data rateper channel by increasing the frequency of the data (TDM). Presentlyinstalled systems typically run at 10 Gigabits-per-sec (Gbit/s) andbelow, however, there is significant progress in developing systems thatoperate at 40 Gbit/s and 160 Gbit/s. The present state of the art is anexperimental system that operates at 1.28 Terabit-per-sec (Tbit/s) forone channel (Nakazawa et al, “Ultrahigh-speed OTDM Transmission beyond 1Tera Bit-Per-Second Using a Femtosecond Pulse Train” WECE Trans.Electron. E38-C, pp. 117-125, (2002)).

There are many technical challenges in increasing the frequency oftelecommunication systems. The one that is relevant here is the opticalsource of the high frequency pulses. The present optical source is a cwlaser diode having its output modulated with a lithium niobate amplitudemodulator. The laser diode can be directly modulated, however, directmodulation of the diode typically imposes a spectral chirp on the laserdiode output that degrades the signal after propagation some distancedown a fiber. It is not certain that lithium niobate modulators and therelated electronics will be able to reach the frequencies and picosecondand subpicosecond pulse widths needed for the systems of the future.Therefore research on alternative sources is presently very active. Thealternative sources can be categorized into three areas. The first twoare laser diode based devices where the thrust of the research is toimprove the fidelity of the pulses generated. One category of suchdevices are mode-locked laser diodes where the frequency is determinedby the round trip time of the laser cavity. The other category of laserdiode based devices is gain switched laser diodes where the frequency isdetermined by the electronics. In order to get short pulses from gainswitched diodes, the pulses need to be compressed after the diodes. Thisis normally accomplished by soliton compression in fibers.

The third category of sources are mode-locked fiber lasers. Mode-lockedfiber lasers generally give high quality pulses but operate at lowerfrequencies than 40-160 GHz. The reason for the lower repetition rate isthere is normally only one pulse in the cavity of a mode-locked laserand the cavity of the fiber laser needs to be long for sufficient gainfrom the fiber. The thrust of mode-locked fiber laser research is toincrease the frequency of these devices by methods such as higherharmonic mode locking.

The configuration that is related to the present invention is the gainswitched diode followed by a fiber for soliton pulse compression. Anearly example is in (Ahmed et al, “Generation of 185fs pedestal-freepulses using a 1.55 μm distributed feedback semiconductor laser”Electronic Letters 31, pp 195-196, (1995)). The potential to generatevariable and low rep rates from gain switched diodes with external pulsecompression would be advantageous for using these devices in high-energysystems. The soliton pulse compression technique normally used isadiabatic soliton compression in dispersion decreasing fiber. Adispersion decreasing fiber is a fiber that has its core slowlydecreased. In order to remain a soliton with decreasing dispersion thepulse width must slowly decrease. Pulse compression in dispersiondecreasing fiber usually gives good pulse quality and pulse compressionfactors up to 16. A disadvantage with the present telecom gain-switcheddiode designs is due to the low pulse energies required and generated.Under these conditions, the nonlinearity for fiber pulse compression issmall and so the fibers are usually quite long and expensive,particularly if the fiber is dispersion decreasing fiber. Often a fiberthat corrects the chirp of the laser diodes is also required before thedispersion decreasing fiber. This is often near a kilometer long. Inaddition, a nonlinear optical device is also often required to remove along pulse pedestal after the fiber pulse compressor. Such a device isdescribed in (K. Tamura et al, “50 GHz repetition-rate, 280-fs pulsegeneration at 100 mw average power from a mode-locked laser diodeexternally compressed in a pedestal-free pulse compressor” OpticsLetters, 27 pp. 1268-70 (2002)) This three element compressor inaddition to the diode makes these systems expensive.

The desired properties for the optical sources of this invention are theability to produce picosecond and subpicosecond pulses with variablerepetition-rates and with energies suitable for further amplification tocreate energetic, ultrafast pulses. Another desired feature is low cost.These sources will be used in ultrafast sources that have manyapplications. A few of the applications now being pursued arefemtosecond micromachining, refractive index alteration in transparentmaterials for optical memory and photonic devices, three-dimensionalintegrated optics and photonic crystals, eye surgery, dentistry,dermatology and microsurgery. For these applications, the pulsecharacteristics are quite different than for telecom systems. Instead ofpicojoule pulse energies and >1 GHz repetition rates, pulse energies inthe microjoule to millijoule range are desired with repetition ratesfrom 1 kHz to 1 MHz. Chirped pulse amplification is used to accommodatethe high energies in the fiber amplifier. In chirped pulse amplificationthe pulse is first spectrally chirped and thus temporally lengthened tokeep the peak power lower in the fiber during amplification. Afteramplification, the pulse is recompressed. Chirped pulse amplification infibers is described (Galvanauskas, “Method and Apparatus for generationhigh energy ultrashort pulses” U.S. Pat. No. 5,400,350). The source inthis patent is a laser diode that is electronically chirped to give a 1ns pulse that was amplified and then compressed to ˜2 ps. It is highlydesirable to be able to obtain even shorter pulses. The source of pulsesfor chirped pulse amplification has been predominately femtosecondmode-locked fiber or solid-state lasers that operate at 50-100 MHz.These sources are typically down-counted; e.g., for 1 kHz operation, onepulse out of 50,000-100,000 is amplified. A source that could beoperated at variable and lower frequencies would be more suitable.

Femtosecond mode-locked fiber lasers do not normally have sufficientpulse energies and the pulses are often longer than desired for thesenontelecom sources. Soliton compression (narrowing) during amplificationin a fiber amplifier and higher-order soliton compression have alreadybeen utilized with these sources. Soliton narrowing during amplificationis equivalent to decreasing the dispersion in the fiber. As the pulsepeak power is increased the pulse width needs to decrease to maintainthe soliton. Such pulse compression for higher-energy pulses isdescribed in (Fermann, Galvanauskas and Harter, “Apparatus and Methodfor the Generation of High-Power Femtosecond Pulses from a FiberAmplifier” U.S. Pat. No. 5,880,877). The fiber amplifier can be lessthan a meter and soliton compression is built into this amplifier. Apulse is seeded into a fiber amplifier and it is amplified to energiesof higher order solitons. As the higher order soliton propagates in afiber its pulse width is periodic. During this periodic evolution, thepulse initially contracts by a factor dependent on the order of thesoliton. It is this phenomenon that is used for compression. Acompression factor of 100 can be obtained by this method but typically asmaller factor is used since pulse energy and length becomes toosensitive. A means to get to even higher pulse energies with solitoncompression is described in U.S. Pat. No. 5,880,877. Higher energies arepossible by utilizing a multimode fiber to propagate a single transversemode. The intensity in the fiber is decreased since the multimode fiberhas a larger mode area for the fundamental mode compared to a singlemode fiber. Thus, higher pulse energies are necessary before solitoneffects again become important.

An alternative to soliton compression in an optical fiber with negativegroup velocity dispersion (GVD) is pulse compression with a fiber havingpositive GVD. Just as with soliton compression, there is a balance ofdispersion with self-phase modulation in the fiber. There issimultaneously spectral broadening of the pulse by self-phase modulationwith temporally stretching of the pulse to give a linear spectral chirpby dispersion. After the fiber, the chirped pulse is recompressed. Thefirst experiments in compression of ultrashort pulses with opticalfibers were accomplished in this manner. In the first experiment by(Nakatsuka et al, “Nonlinear Picosecond-Pulse Propagation throughOptical Fibers with Positive Group Velocity Dispersion”, Physical ReviewLetters 47, pp. 910-913 (1981)), 5.5 picosecond pulses from amode-locked dye laser were compressed to 2 ps giving a compressionfactor of 2.75. In the following six years significant progress was madein pulse compression utilizing this method until (Fork et al,“Compression of optical pulses to six femtoseconds by using cubic phasecompensation” Optics Letters 12 pp. 483-5 (1987)) pulses were compressedto the long-standing record of 6 femtoseconds. The maximum pulsecompression demonstrated is around 110× in one stage of compression.(Dianov, “Generation of high-contrast subpicopulses by single-stage110-fold compression of YAG:Nd³⁺ laser pulses”, Soviet Journal ofQuantum Electronics, 17, pp. 415-416, (1987). Compression factors ashigh as 450 have been reported with a two stage fiber-grating compressorin (Zysst et al, “200-femtosecond pulses at 1.06 μm generated with adouble-stage pulse compressor” Optics Letters 11 pp. 156-8 (1986)). Thiscompression method has been commercialized for pulse compression of cwmode-locked Nd:YAG lasers operating at 1.06 μm. The pulse widths fromthese lasers are between 30-100 ps and the pulses are compressednormally by a factor of 100 to the subpicosecond range. The details ofthese systems can be found in (Kafka et al, “Pulse compression” U.S.Pat. No. 4,750,809, Kafka et al, “Peak power fluctuations in opticalpulse compression”, U.S. Pat. No. 4,896,326 and Kafka et al “Opticalfiber for pulse compression” U.S. Pat. No. 4,913,520). This compressionmethod has not been applied to gain switched diodes for telecom systemssince fibers at the telecom wavelengths (˜1.5 μm) are not normallypositively dispersive, and soliton compression is less sensitive toamplitude fluctuations and does not require additional gratings forcompression. However, the most important factor is the required peakpower. For this compression method orders of magnitude higher pulseenergies are required compared to soliton compression.

More recently, pulse compression utilizing positively dispersiveamplifying fiber has generated nanojoule-range pulse energies fornon-telecommunication applications. One method is described in U.S. Pat.application Ser. No. 09/576,772. This application describes primarilythe use of parabolic pulse amplification. It does describe some pulsecompression (2-10×) for seed pulses with a pulse width of 0.2-1 ps. Itdoes not describe the pulse compression of pulses longer than 1picosecond such as are generated from laser diodes or microchip lasers.(M. E. Fermann, A. Galvanauskas and D. Harter, “Single-mode amplifiersand compressors based on multimode optical fibers”, U.S. Pat. No.5,818,630) also teaches using positive GVD MM amplifier or undopedfibers for pulse compression. It does not teach parabolic pulseamplification or pulse compression for the pulses from gain switchedlaser diodes or microchip lasers that are the initial sources for therather long pulses. For higher energies pulse compressors using positiveGVD fibers that are multimode have been used. Pulse compressorsutilizing multimode, positive GVD graded-index fibers has been used forinitial pulse energies as high as 2 microjoules in (Damm et al,“Compression of picosecond pulses from a solid-state laser usingself-phase modulation in graded-index fibers”, Optics Letters 10, pp.176-8, (1985)). However, in this case, the output was multi-transversemode. Pulse compressors utilizing multimode, positive GVD fibers withsingle-mode output are described in (Fermann and Harter, “Single-modeamplifiers and compressors based on Multi-mode fibers”, U.S. Pat. No.5,818,630). The highest pulse energy utilized in compression with singlemode operation of multimode fibers was in the nanojoule regime.

The sources of the initial pulses in this invention are gain switchedlaser diodes and microchip lasers. Gain switched laser diodes aredescribed in the previously mentioned paper by Ahmed et al. A microchiplaser is a small diode pumped solid-state laser. The microchip is eitheractively Q-switched or passively Q-switched. A commonly used passivelyQ-switch design is given in (Zayhowski et al., “Diode-pumped passivelyQ-switched picosecond microchip lasers”, Optics Letters 19, pp. 1427-29(1994)). The microchip laser that was used in this invention isdescribed in (Zayhowski et al., “Coupled-cavity electro-opticallyQ-switched NdYVO₄ microchip lasers”, Optics Letters 20, pp. 716-8(1995)). Pulse widths as low as 115 ps have been demonstrated for thisactively Q-switched laser. In (Braun et al., “56-ps passively Q-switcheddiode-pumped microchip laser”, Optics Letters, 22 pp 381-2, (1997)) 56ps pulses from a passively Q-switched laser were obtained.

SUMMARY OF THE INVENTION

One object of this invention is to convert relatively long pulses fromrep-rate variable ultrafast optical sources to shorter, high-energypulses suitable for sources in high-energy ultrafast lasers. Anotherobject of this invention is to take advantage of the need for higherpulse energies at lower repetition rates so that such sources can becost effective.

A gain switched laser diode as is used in telecom systems can be used asthe initial source of pulses. In this case, the diode is operated at amuch lower repetition rate. The pulses are still amplified in fiberamplifiers. Fiber amplifiers can be used as constant output powerdevices. The upper-state lifetime in typical doped amplifier fibers suchas Ytterbium and Erbium is in the millisecond range so that theseamplifiers can amplify pulse trains with the same efficiency atrepetition rates from 10's of kHz to 100's of GHz and beyond. If theamplifier is amplifying pulses at 10 kHz rather than at 10 GHz atconstant power, then the pulse energy will be six orders of magnitudehigher. Again, with such high peak powers, pulse compression methodsneed to be different and unique. A first embodiment uses conventionalcompression by spectral broadening the pulses in an optical fiber withpositive group velocity dispersion (GVD) and then compressing the pulsewith diffraction gratings. The object of the pulse compression is toconvert the 3-25 picosecond pulses from the gain switched laser diode topulses that are, subpicosecond.

The second source starts with pulses from a low cost Q-switchedmicrochip laser. These lasers give pulses as short as 50 picoseconds buttypically 250 picoseconds to 1.0 nanosecond. The pulse peak powers aretypically 1-10 kW with pulse energies 6 orders of magnitude higher thanfrom telecom laser diodes. Microchip lasers could be a very costeffective source for pulses less than 10 picoseconds with suitable pulsecompression methods. Single mode fiber compression has thus far beenlimited to pulses shorter than 150 ps and peak powers less than 1 kW.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an example of an output from a single mode silica fiberhaving a laser pulse propagating therethrough.

FIG. 1B shows the Raman gain curve in silica.

FIG. 2 is an illustrative diagram of a first embodiment of the presentinvention.

FIG. 3 is an illustrative diagram of a second embodiment of the presentinvention.

FIG. 4 is an illustrative diagram of a third embodiment of the presentinvention.

FIG. 5 is an illustrative diagram of a fourth embodiment of the presentinvention.

FIGS. 6A and 6B are illustrative diagrams of a sixth embodiment of thepresent invention.

FIG. 7 is an illustrative diagram of a seventh embodiment of the presentinvention.

FIG. 8 shows an example of the spectral output at the output ofamplifier illustrated in FIG. 7

FIGS. 9A and 9B show an example of a pulse according to the seventhembodiment, compressed with a bulk grating.

FIG. 10 is an illustrative diagram of an eighth embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

By way of example, eight preferred embodiments of the present inventionare described herein. The first five embodiments use a laser diode asthe source of long pulses for these ultrafast sources. The remainingembodiments use a microchip laser for the source of the long pulses.

The initial source of pulses is a laser diode operating in the telecomwindow at 1550 nm. This laser diode can be a diode used normally as atelecom pulse source. It can be internally modulated, gain switched, orexternally modulated by utilizing a lithium niobate orelectro-absorption modulator. The pulses need to be in the 1-100picosecond range. The major difference in operation of this laser fromthe use in telecom systems is that the repetition rate will be in thekHz to MHz range rather than the GHz range.

The pulse is then amplified in a fiber amplifier. This amplifier usespositive dispersion fiber at least for the later section of theamplifier. The amplifier is designed so that pulse compression willoccur by means of spectral generation along the fiber as the pulse istemporally chirped. At the end of the fiber, the pulse is typicallyrecompressed with a diffractive element such as diffraction gratings,prisms, or a fiber grating. If the source is to be further amplifiedthen chirped pulse amplification will be used so no pulse compression isneeded at this point or the pulse may need to be further chirped using adiffractive element before amplification.

The design of the fiber compressor can be found with the followingequations. These equations can be found in Govind P. Agarwal, NonlinearFiber Optics, Academic Press Inc. Boston 1989 Chapter 6Z ₀ =πT ² ₀/2|β₂|N ² =γP ₀ T ² ₀/|β₂|Z _(opt) /Z ₀≈1.6/Nγ=2πn ₂ /λA _(eff)1/F _(c)≈1.6/N

Z₀ is the soliton period or in this case where solitons do notpropagate, it is the propagation distance in a fiber with agroup-velocity dispersion parameter of β₂ where a pulse of temporalwidth T₀ doubles in temporal width. N would be the Soliton order for apulse with peak power P₀ in a fiber with a nonlinear coefficient, γ. γis a function of the wavelength of the pulse λ, the nonlinear refractiveindex coefficient, n₂, of the material in the fiber and the effectivearea, A_(eff), of the guided light in the fiber. F_(c) is thecompression factor.

In the design of these fiber compressors, it is necessary to avoid theconversion of the pulse to other frequencies by Raman generation. Thisactually limits the pulse compression factor to about 100. A somewhathigher compression factor of 130 was obtained in the Raman regime (seeKuckkartz et al, “Operation of a fiber-grating compressor in the Ramanregime” Journal of the Optical Society B, 5, pp. 1353-1359 (1988)). Atsome point the stimulated Raman converts the pulse to a longerwavelength, reduces efficiency and prevents higher compression factors.The Raman gain curve in silica can be seen in FIG. 1B (Govind P.Agarwal, Nonlinear Fiber Optics, Academic Press Inc. Boston 1989 FIG.8.1). Stimulated Raman shifts the output spectra by 12 THz. FIG. 1Ashows the output from a single mode silica fiber when a one microjoule,250 ps pulse from a 1.064 nm microchip laser propagates in 100 meters ofsingle mode fiber. There is spectral broadening at 1.065 μm. Themultiple-lobed spectrum is indicative of self-phase modulation. However,significant energy is converted to the first Stokes Raman wavelength at1.12 μm and the second Stokes Raman wavelength at 1.18 μm. The peakpower limits can be calculated. The Raman threshold is a peak powereffect and is determined by:P _(thresh)=16A _(eff) /g _(raman) L _(eff)

Stimulated Raman will grow from noise when the gain is equal to exp(16)and the gain coefficient for Raman is g_(raman)L_(eff)P_(thresh)/A_(eff)where g_(raman) is the coefficient for Raman gain and is 3.5×10⁻¹² cm/Win silica glass, A_(eff) is the mode area, P_(thresh) is the thresholdpeak power and L_(eff) the effective length of the fiber. The effectivelength is modified in a high gain amplifier since the peak power is onlyhigh near the end of the fiber. The formula is:L _(eff)=1/g(1−exp(−gL))where g is the gain coefficient of the fiber amplifier and L is thephysical fiber length. Increased performance can be obtained byutilizing fibers designed to prevent propagation of longer wavelengthssuch as reported in (Arbore et al, “S-band Erbium-Doped Fiber Amplifiersfor WDM Transmission Between 1488 and 1508 nm” in OSA Trends in Opticsand Photonics (TOPS) Vol. 86, Optical Fiber Communication Conference,Postconference Edition (Optical Society of America, Washington, D.C.,2003), pp. 374-376.) The fiber described in this reference is a“depressed-cladding design” to create a fundamental-mode cut off abovethe wavelength of operation. Another solution to provide loss to longerwavelengths to prevent Raman generation is a series of filters along thelength of fiber. (Etsuko Ishikawa et al, in Proceedings of ECOC 2001(2001), Post-Deadline paper). These fibers can also be used in fiberamplifiers to obtain higher peak powers.

In the first embodiment shown in FIG. 2, a gain switched laser diode asis described in Ahmed et al is used as the source 1. The chirped 27picosecond pulse from DFB laser diode at 1550 nm is recompressed toabout 3.3 ps in a dispersion shifted fiber 2. This is about 1 km longwith D=−26.6 ps/nm/km. The pulse is now about 2× transform limited. Thechirp compensation is sufficiently accurate as is described inMestadagh, D., “Fiber-grating pulse compressor: Performance withinitially chirped pulses” Applied Optics 26, pp. 5234-5240 (1987). Thesepulses are then amplified in an erbium fiber amplifier (EDFA) 3 with again of approximately 30 db up to 2 kW peak power. The amplifier isdesigned so little self-phase modulation takes place in the amplifier.This can be accomplished with a short fiber length (˜10 M) and a largeeffective area of the mode (˜100 μm²). The rest of the fiber pulsecompressor consists of an undoped fiber 4. The primary constraint onthis fiber is positive GVD. One example is the Lucent Ultrawave IDF, asis described in (Knudsen, S. “Design and manufacture of dispersioncompensating fibers and their performance in systems”, Proc. of OFC '02,paper WU3, 330-331 (2002)). The optimum length, Z_(opt), of the undopedfiber should be ˜20 M for a compression factor of around 50 and a targetpulse width around 100 fs. The energy of the amplified pulse is 6.6nanojoules. If the rep-rate for this source is 1 MHz, then the averagepower from this amplifier is a modest 6.6 mW.

With these design parameters it is useful to show that this method isimpractical for telecom systems. If this nonlinear fiber were part of atelecom source at 10 GHz, then the output would be an unreasonable 66Watts. At the same average power of 6.6 mW which is reasonable for atelecom system, the peak power for a 10 GHz telecom source would bereduced by four orders of magnitude. At this peak power, spectralbroadening does not occur in the fiber so the disclosed pulsecompression method would not work.

Returning to the invention, spectral generation occurs by self-phasemodulation in the fiber. Self-phase modulation is proportional to therate of change in the intensity. As the positive dispersion spreads outthe pulse, at the leading and falling edge of the pulse, spectrum isgenerated. Blue shifted components of the spectrum are generated at thetrailing edge of the pulse and red shifted components are generated atthe leading edge of the pulse. The pulse ends up being temporallyrectangularly shaped.

The stretched pulse can now be compressed or stretched with a grating 5as is shown in FIG. 2. A circulator 6 is used to transit the pulses intoand out of the fiber grating. The pulse would be stretched in the caseof subsequent amplification by a chirped pulse amplification system, orcompressed for output if there is no subsequent amplification stage.

In past fiber compression systems, the grating for compression has beena bulk grating. One design is described in (Kafka et al, “PulseCompression” U.S. Pat. No. 4,750,809). However, a problem has been thatthe difference from linear group velocity dispersion for bulk gratingsdoes not match that for the stretched pulse. (Tomlinson et al, “Limitsof fiber-grating optical pulse compression” JOSAB 4 pp. 1404-11,(1987)). It has been shown that fiber gratings are a better match thanthe bulk gratings in (Williams et al, “The compression of optical pulsesusing self-phase modulation and linearly chirped Bragg-gratings infibers” IEEE Photonics Technology, pp. 491-3, (1995)). The fiber gratinghas several other advantages. In (Kafka et al, “Peak power fluctuationsin optical pulse compression” U.S. Pat. No. 4,896,326) it has been shownthat amplitude fluctuations can lead to pulse width fluctuations. Thispatent describes a feedback circuit for controlling the input laser forcorrecting these fluctuations. However, pulse width fluctuations couldbe corrected by varying the chirp of the fiber grating. This method isbeing applied for correcting dispersion fluctuations in telecom systemsas is described in (Kwon et al “Group-delay-tailored chirped fiber Bragggratings using a tapered elastic plate” IEEE Photonics TechnologyLetters, 14, pp. 1433-1435, (2002)). In this work the chirp is tailoredby adjusting the strain on the fiber grating. Another means of adjustingthe chirp is by adjusting the temperature along the grating. Anotheradvantage is that the bandwidth of the gratings can be designed to actas a filter. It has been shown that by cutting the extremes of thespectra that pulse quality can be improved in this pulse compressionmeans. ((Heritage, “Spectral windowing of frequency-modulated opticalpulses in a grating compressor”, Applied Physics Letters, 47, pp. 8789(1985)).

Various methods for removing a pedestal from short pulses can also beapplied in this embodiment of the present invention. Such methodsinclude frequency doubling, the use of a nonlinear loop mirror, andnonlinear polarization rotation in a fiber and in hollow waveguidesfilled with a noble gas.

In the second embodiment as is shown in FIG. 3, the short pulse from thelaser diode is generated by a method other than gain switching the laserdiode. The technique employed here is an external lithium niobatemodulator 32 after a cw laser diode 31. The advantage with this methodis that little chirp is imparted on the pulse so that chirp compensationis not necessary. This method is commonly used in the telecommunicationsfield. However, one specification that is more critical for thisapplication than for telecommunications is the extinction ratio. Thetypical extinction ratio for a lithium niobate modulator is 20-30 db.This specification can be found from the data sheet for JDSU model no.21013142. If this modulator is turned on to give a 100 ps pulse at a 10kHz pulse rate, then the pulse is on for 100 ps while the cw is on for0.1 ms. This means that the cw component is on the order of 10⁵ longerthan the pulse. If the cw amplitude is cut by just 30 db, the averagepower will still be dominated by this cw component. At higher repetitionrates two modulators can be used and will be sufficient. Anotherapproach is to gate the cw laser and have it on for a short time duringthe pulse. If the laser diode is gated on for about 3 ns while themodulator is switched then the extinction ratio is not critical. Anyother means to create a short pulse from a laser diode can beconsidered. Another possibility is an electroabsorption modulator (notshown) after the diode.

In this embodiment (FIG. 3), the pulses are amplified in an Ytterbiumdoped fiber amplifier (YDFA) 33. The compressor fiber 34 is the samefiber as in the first embodiment but approximately 200M long, comparedto 20M. The fiber is longer than that in the first embodiment for tworeasons. The first is the initial pulse width is an order of magnitudelonger. The second reason is that the peak power is lower. Since theinitial pulse width is longer, it may be desirable to have a secondstage of compression. This is easily implemented in the fibergrating/circulator configuration 35 shown in FIG. 3. The fiber outputfrom the circulator can be designed as the second compression fiber. Thefirst stage was designed for a compression factor of near 80 so pulsesaround 0.5 picoseconds are expected. The second stage will be designedfor 10× additional compression. This design can be implemented by addingan additional of 40 cm of the same fiber as was used in the firstcompression stage. The amplitude of the pulse will need to becontrollable and lowered by about a factor of 4. To this end, anattenuator can be placed in the circulator. The pulses can be usedstretched for further amplification, or compressed by an external bulkgrating after this second fiber compressor.

Another configuration for a two-stage compressor is shown in the thirdembodiment in FIG. 4. It utilizes a four-port circulator 45 with anadditional fiber grating 47. The second fiber compressor is spliced infront of the first fiber grating. The first pass through the extra 40 cmof fiber has negligible effect since the pulse is stretched. After thesecond pass through this compressor fiber, additional spectrum isgenerated and the pulse can be further compressed in the second chirpedfiber grating.

The forth embodiment of this invention (shown in FIG. 5) includes anadditional amplifier 58. It is desirable for the amplifier to operate asa parabolic pulse amplifier. The use of parabolic pulse amplificationhas unique advantages after the pulse compressor fiber 34. Theadvantages come from the fact that as the pulse energy increases at agiven rate with propagation, the peak power increases at the square rootof that rate because the spectral and temporal width is increasing atthe square root of that rate. Thus, higher pulse energy is possiblebefore the threshold of Raman generation. Also, since the pulse from theparabolic pulse amplifier is further spectrally broadened and iscompressible, further compression is possible after the compressionfiber. It is normally considered necessary to recompress the pulse afterthe pulse compression fiber for further compression of the pulse. Thisis the reason for the two stage compressor in the third embodiment. Aparabolic pulse amplifier can replace one of the stages of compressionand alleviate the need for a second grating compressor. This is adistinct advantage.

Another configuration within this embodiment is to replace the twoytterbium amplifiers and the 200 meters of compression fiber with asingle amplifier and 100 meters of pulse compression fiber connected tothe amplifier. This amplifier and pulse compression fiber is doublepassed in this variation.

In order for the system to include a parabolic pulse amplifier, it isnecessary to transform the pulse shape from the laser diode into a pulsewith a parabolic temporal shape and a linear spectral chirp. The firststep in this process is the amplification of the pulse in a doped fiberto a sufficient pulse amplitude such that self-phase modulation takesplace in the fiber. The self-phase modulation needs to be sufficientlystrong that the pulse profile transforms from being the same spectrumalong the temporal profile, to the spectrum being a function of time.The final transformation is the temporal profile change from that of theinitial pulse, to near parabolic, this caused by more energy beingaccumulated where the self-phase matching is the strongest. This processdoes take place in the undoped fiber in the first embodiment and thepulse shape from this undoped fiber is sufficiently close to a chirped,parabolic pulse to be the input into the parabolic pulse amplifier. Ithas been shown theoretically and experimentally that the pulse does notneed to be a perfect parabolic pulse. Thus, the first section of theamplifier is a linear amplifier without nonlinear effects. The secondsection utilizes self-phase modulation and dispersion to transform thepulse into a pulse shape close to parabolic with linear chirp. Thissection can be an amplifier or an undoped fiber. The final section isthe parabolic pulse amplifier.

This embodiment of the invention can be operated near 100 MHz so as toreplace the standard Nd:YAG or Nd:Vanadate mode-locked laser, or thenewer mode-locked lasers such as in (Kafka et al, U.S. Pat. No.6,421,573, “Mode-locked laser and amplifier”).

To obtain different wavelengths of output frequency, conversion crystalscan be used after this laser. Thus this laser can be used in the UV fordetecting flaws on a surface as in U.S. Pat. No. 6,373,565 “Method andapparatus to detect a flaw in a surface of an article”, or to repair adefect in an electronic device as in U.S. Pat. No. 6,436,602 “Method ofrepairing a defective portion in an electronic device” or forphotolithography as in U.S. Pat. No. 6,421,573 “Quasi-continuous Wavelithography Apparatus and Method”. Thus, laser can be used inreplacement of most types of mode-locked lasers with the advantage ofbeing synchronizable to an event. For example, in repairing a flaw, oncethe beam is positioned to the right spot, a pulse or train of pulses canbe fired.

A fifth embodiment uses a laser diode that is spectrally and temporallychirped electronically. The pulse source being temporally chirped in thetime domain by electronic means is described in U.S. Pat. No.5,4000,350, and details on optimizing the electrical tuning of the laserdiode is described in U.S. Pat. No. 5,633,885. The pulse shape from thisdiode is sufficiently close to a parabolic pulse shape that they can beamplified in a parabolic pulse amplifier. This will generate additionalspectrum so the pulses can be compressed to below the 2 ps that wasobtained previously.

A sixth embodiment of the invention is shown in FIG. 6A and includes aQ-switched microchip laser 60 (as shown in FIG. 6B) as the pulse source61 and a compression fiber 62. In the cases where the pulse width isshort the lasing material has been Nd:Vanadate. The lasing wavelength ofNd:Vanadate is 1064 nm. The microchip laser is a monolithic piece, wherethe lasing material is bonded to the Q-switching mechanism. The cavitylength measures around one millimeter. Thus, a Q-switched laser can givepulse widths as short as a cw mode-locked Nd:YAG laser and can replacethe mode-locked lasers for the source where fibers are used forcompression. One of the advantages with the microchip laser is that therepetition rate can be varied. It can have a low rep rate so the pulseenergy is on the order of 1 microjoule compared to nanojoules from themode-locked laser. Another advantage with this microchip laser is thecavity is monolithic so the output pointing stability is very good, andthe alignment to a single mode fiber is robust. This was a large problemwith systems based on cw mode-locked lasers.

The microchip laser in this embodiment has a pulse width of 250 ps. Withthis pulse width, a design similar to the earlier embodiments is notpossible, unless a fiber is used that inhibits Raman generation. Thereis a different design point for longer pulses and shorter fibers whereGVD is negligible. This was studied theoretically in (Tomlinson et al,“Compression of optical pulses chirped by self-phase modulation infibers” Journal of the Optical Society of America B, pp. 139-149,(1984)). A product was based on this design and is described in (U.S.Pat. No. 4,913,520, U.S. Pat. No. 4,896,326 and Kafka et al “Peak powerfluctuations in optical pulse compression”, IEEE Journal of QuantumElectronics 24, pp. 341-50 (1988)). The design for this compressionfiber can be facilitated with the equations in (Govind P. Agarwal,Nonlinear Fiber Optics, Academic Press Inc. Boston 1989 Chapter 4). Themaximum phase shift, Φ₀, is at the peak of the pulse with a peak powerof P₀.Φ_(max)=γP₀L_(eff)γ and L_(eff) are defined above. For a Gaussian shaped pulse that is agood approximation of the microchip pulse, the ratio of spectral widthgenerated, δω_(max), to the initial pulse, Δω, width is:δω_(max)/Δω=0.86 Φ_(max)

If the initial pulse is transform limited, and the self-phase modulatedpulse can be compressed near-transform limited, then this ratio is thecompression factor.

Thus in the sixth embodiment (FIG. 6) the microchip laser is a singlelongitudinal Nd:vanadate source that provides a smooth temporal profile.The pulse width is 250 picoseconds. One solution for the compressionfiber 62 is a standard single mode fiber with a mode field diameter of5.9 μm and a NA of 0.12. The length of this compression fiber would beabout 2 meters for a compression ratio of around 50. The output energyfrom microchip lasers can be 10 microjoules. In this case, the lightintensity at the entrance face of the fiber will be near the damagethreshold. A coreless end cap (not shown) can be used on the fiber sothe mode can expand before the surface of the fiber. Otherwise, anamplifier with a larger mode field diameter can be used, such as amultimode fiber that propagates a single mode or a holey fiber amplifieras was used in (Furusawa et al “Cladding pumped Ytterbium-doped fiberlaser with holey inner and outer cladding”, Optics Express 9, pp.714-720, (2001)). If a fiber with an order of magnitude higher mode area(mode field diameter of 19.5 μm) is used, then the parameters in thefiber will be the same as in the case with 1 microjoule input. So thefiber length would again be 2 meters.

Since there is no interplay between dispersion and self-phase modulationin this design, the pulse compression can take place simply inamplifiers. For pulse energies significantly greater than 1 microjoule,the single mode beam should be further amplified in a multimode fiber.If further amplification were desired, it would be preferable to keepthe pulse stretched. This chirped pulse source would be ideal foramplification of ultrashort pulses by chirped pulse amplification. Thepulse is then compressed after amplification. This is the seventhembodiment as shown in FIG. 7. Here, the microchip 71 was operated at0.5 μJ, and produced 250 ps, pulses at 6 kHz. The amplifier fiber 72 wasa multimode amplifier fiber that amplified a single mode with amode-field diameter of 17 μm. The pulse was then amplified to 30microjoules where Raman limited the amplification. The spectral outputis shown in FIG. 8 at the output. The input spectral width (0.007 nm) isbelow the resolution limit (0.1 nm) of the spectrum analyzer. This casewas numerically modeled and it was found that this pulse should compresswith bulk gratings to ˜7 ps as is shown in FIGS. 9A and 9B. The dottedlines show the results of spectrally filtering the output to reduce thepedestal. In the embodiment shown in FIG. 7, a compressor 73 may also beemployed as an option.

An eighth embodiment, illustrated in FIG. 10, utilizes the sameamplifier fiber 103, as in the seventh embodiment, however, in a doublepass configuration. Double pass configurations can be used for theamplifiers and the pulse compression fibers to maintain singlepolarization operation for non-PM fiber. Also, the amplifier can operatein a lower gain and higher power mode. The amplifier fiber can be usedas a pulse compression fiber in both directions. If this is the case, afiber grating may be desired in place of the mirror 104.

The fiber grating can be used to control the temporal shape of thepulse. The pulse is now chirped, therefore changing the reflectivity ofthe fiber grating as a function of wavelength will shape the pulse.Thus, by changing the shape of the pulse, the linearity of the chirp ofthe output can be changed to better match that of the bulk gratings.This method has been described by adding a amplitude mask in a bulkgrating in (Thurston et al, “Analysis of picosecond pulse shapesynthesis by spectral masking in a grating pulse compressor” IEEEJournal of Quantum Electronics, QE-22 pp. 682-696 (1986)).

The seventh and eighth embodiments, which include a microchip laser andan amplifier will exhibit output powers greater than 500 nJ.

An application for the pulse compressed microchip laser is to pump again switched laser. In (Harter et al, “Short pulse generation fromTi:doped Materials” Digest of Conference on Lasers and Electro-Optics(Optical Society of America, Washington, D.C. 1988), p 466-7) a 100-200ps pulse could be generated from a gain switched Ti:sapphire laser. Thislaser was pumped by a 5 ns pulse from a Q-switched, frequency-doubledNd:YAG laser. This work was referenced and described in (Zayhowski etal, “Gain-switched pulsed operation of microchip lasers” Optics Letters,14, pp. 1318-20). In this work, the gain-switched Nd:YAG laser thatgenerated 80 ps pulses, and was pumped with a gain switched Ti:sapphirelaser that was pumped with a Q-switched, frequency doubled Nd:YAG laser.More recently in (Zayhowski “Laser system including passively Q-switchedlaser and gain-switched laser”, U.S. Pat. No. 6,400,495) a gain switchedTi:sapphire laser that is pumped with a passive Q-switched, frequencydoubled Nd:YAG laser is claimed. There are no experimental results, butthe document points out that the pump pulse should be shorter than 5×the pulse from the gain switched laser. The reason for this ratio ofpulse widths can be intuitively understood. The build-up time for a gainswitched (Q-switched) laser is about 10× its pulse width. So if thepulse width of the pump laser is 5× longer than the gain switch pulse,all the energy goes into that single pulse. In the Harter paperreferenced above, the pump pulse was 25×-50× longer than the gainswitched pulse. In this work, if the laser is pumped hard, multiplepulses were obtained. Also, the gain switch pulse came out at the peakof the pump pulse so about half of the pump pulse energy is wasted.

In order to get pulses that are shorter than 100 picoseconds, a shorterpump pulse is highly desirable. For pulses shorter than 10 ps and,particularly, femtosecond pulses, the pulses from the typical microchipare definitely too long for the pump laser. To get pulses shorter than10 ps, the pulse-compressed microchip laser can be used as a pump.However, to get subpicosecond pulses from a gain switched laser requiresmore than just a short pump pulse. The time it takes light to travelaround the laser cavity must be shorter than the required pulse width.Thus, to get less than 10 picosecond pulses, the cavity would need to beshorter than c×2×L×n, where c is the speed of light, L is the cavitylength and n is the refractive index of the material in the cavity.Thus, for the Ti:sapphire gain switched laser in the Harter paper, thelength of the crystal would need to be at least an order of magnitudeshorter than the 5.5 mm used and, thus, 500 μm or less. However, forthis thickness it is difficult to absorb pump light. The problem withabsorbing pump light in a very thin laser material has been solved in(Brauch et al, “Laser Amplifying System” U.S. Pat. No. 5,553,088). Theamplifier design uses a very thin material that has good absorption.However, with very thin discs, multiple passes of the pump is necessary.Table 1 in U.S. Pat. No. 5,553,088 gives a list of materials withsuitable absorption numbers. Ti:sapphire(Ti:Al₂O₃) is one that islisted. In order to use a 500 μm disc, the pump will need to make 8passes through the material. In U.S. Pat. No. 5,553,088, the eighthembodiment illustrated in FIG. 28 shows a configuration with 8 passesthrough the material. Eight passes is a complex device, fewer doublepasses would be more reasonable. The additional constraint is that thetime to transit the optical path length needs to be on the order of thepump pulse width. With a short cavity, it is difficult to obtain a largediameter mode. The repetition rate is low so that the overall thermalload will be low, so thermal lensing will not help to support a largediameter single mode. The use of longer cavities and thermal lensing inorder to support large modes in thin semiconductors is described in(Mooradian et al, “Optimized laser energy conversion through automaticmode matched pumping” U.S. Pat. No. 5,627,853). However, the gain inthis gain switched laser is significantly higher than that in cw lasing,so that gain guiding will take place and support a large diameter singlemode. The use of gain guiding to support large transverse modes is usedin fibers in U.S. Pat. No. 5,818,630.

To summarize, the invention has the following attributes:

-   -   Use of pulse compressed microchip as a pump to get high energy        pulses less than 10 ps from a gain switched laser.    -   Use of multiple passes for absorption where the temporal path        length is on the order of the pump pulse width.    -   Use of gain guiding for a thin stable flat cavity without        thermal lensing.

While various implementations of variable rep-rate source for highenergy, ultrafast lasers according to the present invention have beendescribed in detail, a skilled artisan will readily appreciate thatnumerous other implementations and variations of these implementationsare possible without departing from the spirit of the invention.

For example, the source of pulses can have a large range of pulsewidths. A practical limitation is the grating utilized for compression.One meter long fiber gratings have been made that can compress 10nanosecond pulses. Thus, some implementations of the invention mayutilize pulses of up to about 10 nanoseconds. On the other hand, bulkgrating compressors have been built that compress 4 nanosecond pulses.Accordingly, some advantageous implementations of the present inventionmay utilize pulses which are up to about 4 nanoseconds. Also, atpresent, 10 cm fiber gratings and bulk compressors that compress 1nanosecond pulses are readily available. Thus, some particularlyadvantageous implementations of the invention may utilize pulse widthsup to about 1 nanosecond.

The scope of the invention is defined by the claims set forth below.

1. A combination comprising: a pulse source outputting pulses having apulse width between 1 picosecond and approximately 10 nanoseconds induration at a repetition rate in a range of from 1 kHz to less than 10MHz; an amplifier directly receiving an unstretched output of said pulsesource; and a fiber with net positive group-velocity dispersion (GVD)and self phase modulation receiving an output of said amplifier andspectrally broadening said pulses.
 2. A combination comprising: a pulsesource outputting pulses having a pulse width between 1 picosecond andapproximately 10 nanoseconds in duration at a variable repetition ratein a range of from 1 kHz to less than 10 MHz; and a fiber amplifierwhich includes at least one fiber with net positive group-velocitydispersion (GVD) and self phase modulation receiving an unstretchedoutput of said pulse source and spectrally broadening said pulses.
 3. Acombination comprising: a pulse source outputting pulses having a pulsewidth between 1 picosecond and approximately 10 nanoseconds in durationat a variable repetition rate in a range of from 1 kHz to less than 10MHz; and a fiber amplifier with net positive group-velocity dispersion(GVD) and self phase modulation receiving said pulses directly, asoutput by said pulse source, said fiber amplifier amplifying andspectrally broadening said pulses.
 4. A combination comprising: a pulsesource outputting pulses having a pulse width between 1 picosecond andapproximately 10 nanoseconds in duration at a variable repetition ratein a range of from 1 kHz to less than 10 MHz; and a fiber with netpositive group-velocity dispersion (GVD) and self phase modulationreceiving an unstretched output of said pulse source and spectrallybroadening said pulses, wherein said fiber is arranged in a double passconfiguration.
 5. A system comprising: a pulse source outputting pulseshavina a pulse width between 1 picosecond and approximately 10nanoseconds in duration at a repetition rate in a range of from 1 kHz toless than 10 MHz, said pulse source comprising a light source; and a netpositive group-velocity dispersion (GVD) fiber receiveng an unstretchedoutput of said light source, and causing spectral generation by selfphase modulation in said fiber.
 6. A system comprising: a pulse sourceoutputting pulses, said pulse source comprising a light source; a netpositive group-velocity dispersion (GVD) fiber receiving an unstretchedoutput of said light source, and causing spectral generation by selfphase modulation in said fiber; and an amplifier receiving an output ofsaid positive GVD fiber, said amplifier being a parabolic amplifier. 7.The combination as claimed in claim 1, wherein said source includes alaser diode.
 8. The combination as claimed in claim 1, wherein saidsource is internally modulated.
 9. The combination as claimed in claim1, further comprising means for removing a pedestal from pulses outputfrom said fiber with positive GVD.
 10. The combination as claimed inclaim 7, wherein a pulse shape output from said diode is sufficientlyparabolic for amplification in a parabolic pulse amplifier.
 11. Thecombination as claimed in claim 2, wherein said fiber is a multimodefiber in single mode operation for self phase modulation, and pulseenergy is greater than 500 nJ.
 12. The combination as claimed in claim2, wherein said fiber is a single mode fiber having a length ofapproximately 2 meters for a compression ratio of approximately
 50. 13.The combination as claimed in claim 1, wherein said amplifier is one ofa multimode fiber and a holey fiber amplifier.
 14. The combination asclaimed in claim 2, wherein pulse energies are greater than 1 μJ, saidfiber is a multimode fiber, and said pulses are stretched.
 15. A highpower nonlinear fiber amplification system comprising: a seed sourceproducing pulses with a pulse width shorter than 1 ns and longer than 1ps, a nonlinear fiber amplifier, said amplifier being configured tospectrally broaden said seed pulses in the presence of positivedispersion, gain, and self-phase modulation, and to generatesubstantially linearly chirped pulses with a spectral bandwidth up toapproximately 50 times larger than a spectral bandwidth of said seedpulses and wherein the chirp is substantially generated during or afterthe amplification process.
 16. A high power nonlinear fiberamplification system according to claim 15, further comprising a pulsecompressor following said nonlinear amplifier recompressing the pulsesto substantially shorter duration than the seed source pulses.
 17. Thecombination according to claim 1, wherein said seed source comprises amodulated semiconductor laser.
 18. The combination according to claim 4,wherein said semiconductor laser is gain switched.
 19. The combinationaccording to claim 4, wherein said semiconductor laser is externallymodulated.